Bean rust

tiny pale yellow-white flecks appear on the lower, older leaves, then quickly swell into small raised rusty-brown pustules (bumps) — mostly on the underside of the leaf. Rub one with your thumb and a rusty-orange powder (the spores) dusts your finger — that powder-on-the-thumb test is the sure way to know it is rust and not something else.

each pustule is often ringed by a yellow halo, and the rust climbs from the lower leaves up the plant and onto the pods and stems; late in the season the pustules darken to blackish-brown. A heavy attack turns the leaves yellow and dry and drops them early — the plant looks scorched and the pods stay small.

flat, dry, dead patches that do not dust your finger orange are a different problem. Angular leaf spot makes brown patches boxed in straight by the leaf veins; common bacterial blight makes big brown blotches with yellow edges. Bean rust is raised and powdery; those look-alikes are flat and dead.

start once the plants bush out and the leaves close over the rows, and check hardest from flowering through pod-fill — that is when rust does the most damage. Step up your checks after spells of warm, wet, humid or heavy-dew weather (rust thrives at about 17–27°C with the leaves staying wet for hours).

cross the field in a big "W" so you cover all four corners and the middle — not just the row nearest the path, where you happen to pass every day.

at about 10 stops, turn the lower, older leaves over on a couple of plants — the pustules hide on the underside — and rub any suspect spot to see if rusty-orange powder comes off on your thumb.

the moment you find the first rusty pustules on the lower leaves — even on just 1 plant — from flowering onward in humid weather, act now. Rust throws out wave after wave of spores and doubles fast, so first-sighting is action time; don't wait for it to reach many plants the way you would for an insect pest.

at the very first pustules, pick off and destroy — bury deep or burn — the few spotted lower leaves before they shower the field with spores, and open up the crowded canopy for airflow. Never brush through the plants or pick beans while they are wet, or you will carry spores from plant to plant on your hands and clothes.

if you had no resistant variety and rust is appearing under humid pressure, spray mancozeb. It is a protectant (contact) fungicide — it coats the leaf and blocks new spores from getting in, but it cannot cure spots already there — so start at the first pustules, cover both sides of the leaves (especially the undersides), and repeat every 10–14 days while the weather stays warm and wet. Safety: cover your skin, eyes, nose and mouth; spray in the cool of the day with the wind behind you; keep children, animals, bees, food and water well away; respect the pre-harvest interval (the number of days you must wait after the last spray before you pick or eat the beans — it is printed on the label); and wash yourself and your clothes afterwards. Resistance (the fungus getting used to one chemical until it stops working): if you must spray again and again, alternate with a fungicide from a different group between sprays so the rust cannot get used to it.

keep the protectant schedule tight and ahead of the wet weather, and make sure you switch to a locally-resistant variety next season. But if it is late in the season and most leaves are already rusty, yellow and dropping, a protectant cannot bring dead leaves back — stop wasting money on sprays, harvest whatever pods have filled, then cut and burn all the bean debris so the spores don't carry over. Start next season clean: a resistant variety, rotated land, and wider spacing.